1. Field
The subject matter disclosed herein relates to the suppression of multipath effects for signals received from satellite positioning systems.
2. Information
A satellite positioning system (SPS) typically comprises a system of earth orbiting satellites enabling entities to determine their location on the earth based, at least in part, on signals received from the satellites. Such an SPS satellite typically transmits a signal marked with a repeating pseudo-random noise (PRN) code of a set number of chips. For example, a satellite in a constellation of a Global Navigation Satellite System (GNSS) such as GPS or Galileo may transmit a signal marked with a PRN code that is distinguishable from PRN codes transmitted by other satellites in the constellation.
To estimate a location at a receiver, a navigation system may determine pseudorange measurements to satellites “in view” of the receiver using well known techniques based, at least in part, on detections of PRN codes in signals received from the satellites. Such a pseudorange to a satellite may be determined based, at least in part, on a code phase detected in a received signal marked with a PRN code associated with the satellite during a process of acquiring the received signal at a receiver. To acquire the received signal, a navigation system typically correlates the received signal with a locally generated PRN code associated with a satellite. For example, such a navigation system typically correlates such a received signal with multiple code and/or time shifted versions of such a locally generated PRN code. Detection of a particular time and/or code shifted version yielding a correlation result with the highest signal power may indicate a code phase associated with the acquired signal for use in measuring pseudorange as discussed above.
FIG. 1 illustrates an application of an SPS system, whereby a subscriber station 100 in a wireless communications system receives transmissions from satellites 102a, 102b, 102c, 102d in the line of sight to subscriber station 100, and derives time measurements from four or more of the transmissions. Subscriber station 100 may provide such measurements to position determination entity (PDE) 104, which determines the position of the station from the measurements. Alternatively, the subscriber station 100 may determine its own position from this information.
Subscriber station 100 may search for a transmission from a particular satellite by correlating the PRN code for the satellite with a received signal. The received signal typically comprises a composite of transmissions from one or more satellites within a line of sight to a receiver at station 100 in the presence of noise. A correlation is typically performed over an integration time “I” which may be expressed as the product of Nc and M, where Nc is the coherent integration time, and M is the number of coherent integrations which are non-coherently combined. For a particular PRN code, correlation values are typically associated with corresponding PRN code shifts and Doppler bins to define a two-dimensional correlation function.
FIG. 2 depicts an example idealized correlation function for a SPS signal received along a direct line of sight in the absence of interference from multipath signals. For this example, the SPS signal is a GPS signal. FIG. 3 provides a close-up view of the example correlation function. Peaks of the correlation function are located and compared to a predetermined noise threshold. The threshold is typically selected so that the false alarm probability, the probability of falsely detecting a code phase of a received SPS signal, is at or below a predetermined value. The triangular shape of the correlation function of FIG. 3 indicates that there is little or no correlation between the received GPS signal and the local replica of the code when the code phase offset is greater than about 1 chip in either direction. The bulk of the power in the correlation function of FIG. 3 occurs within the region between +1 and −1 chips offset from the received direct path GPS code signal.
FIG. 4 is a diagram depicting an autocorrelation function for a direct signal 401 combined with a multipath signal 403 with a positive polarity. The resulting composite signal 405 depicts the effects caused by the reflected multipath signal 403 interfering with the direct path signal 401 as the two signals 401 and 403 are received at an antenna and processed at a receiving device. Although FIG. 4 depicts only a single multipath signal, it is common for multiple multipath signals to contribute to distortions of the direct path signal. In general, multipath signals may comprise a signal from a transmitter that reflect from mountains, buildings, etc., before reaching a receiver. Multipath signals are delayed relative to the direct signal due to the increased distance they travel from the transmitter to the receiver relative to the direct path signal. As a result of the reflections, multipath signals are typically lower in amplitude compared to the direct path signal. For this example, multipath signal 403 adds to direct signal 401 to yield composite signal 405.
Although multipath signal 403 for this example is depicted as having a positive polarity with respect to direct signal 401, it is possible for multipath signals to arrive at the receiver with a polarity opposite that of the direct signal. For the situation where the multipath signal has a polarity opposite that of the direct signal, the resulting composite signal will have a reduced amplitude relative to the direct signal due to the cancellation caused by the negative-polarity multipath signal.
As described above, superposition at the receiver of any additional signal onto the desired direct signal from the satellite during the period of time when signal correlation occurs may distort the autocorrelation function and produce an altered correlation function for the composite signal, such as that depicted by composite signal 405 in FIG. 4. These distortions may lead to errors during correlation tracking functions, which may result in errors in pseudorange measurements, and which may further produce errors in estimated location coordinates for the receiver.